Make sure the cylinder locks up properly, meaning when the hammer is ready to fall be sure that the cylinder is locked up, or does not move.

Look for a bent ejection rod.

Be sure the action is smooth, but not too light.

cock the hammer and hold it with the barrel pointing straight up. Place a pencil, eraser side first, into the barrel. Press the trigger and the pencil should jump.

Repeat, only this time tap the trigger with a knife handle or other solid object. If the safety mechanism is working properly and you do not hold the trigger back, the pencil should not jump.

The other issue is cylinder alignment, and I believe a gunsmith needs to check that. That insures that the cylinder holes and barrel are lined up correctly when the hammer falls; otherwise the gun will shave lead and spit it out on those around you.

I was injured at CHL instructor school by the guy next to me when his S&W was shaving lead.

You are looking at the topstrap of a revolver, the top of the gun over the cylinder, where it meets the barrel. Most revolvers experience flame cutting if the cylinder has excessive fore/aft gap where it meets the barrel, some even with the gap in spec if shot with hot loads such as magnums.
Ignore the cracked forcing cone but this pic should illustrate it better.

If you are looking at a Colt cock the hammer, pull the trigger when lowering the hammer. Holding the trigger back try to move the cylinder. If it moves any then it is going out of time or is out of timing. A Colt that has the trigger pulled on the hammer like that will have absolutely zero movement in the cylinder.